So I have a 20 gal (long) tank. I bought the 10-20 gal Zoo-Med reptitherm UTH. It does a crap job keeping the temps up to snuff. I was wondering if I should toss the smaller one and upgrade to the 30-40 gal UTH maybe that would work better since the heat would spread over a larger area.

The 30-40 gal looks like it will run a little less than half the length of the tank and around 2/3 of the width. Hard to tell exactly since its in the package still, don't want to open it till I'm sure I want to use it.

Any advice on what size UTH would be good for a 20gal tank. The overall dimensions are 30" long x 13" wide, give or take a slight bit.

And yes it will be hooked to a thermostat.